The increasing use of the Internet and other computer based communications systems has lead to the development of systems and methods for providing higher bandwidth access over existing telephone lines. A conventional access loop from the Central Office (CO), for example, employs a twisted copper pair and transmission systems implementing technologies such as Digital Subscriber Line (DSL) have been developed to carry both voice and broadband data signal over such access loops.
Due to the low pass filtering characteristics of a standard twisted pair telephone wire, the amplitude of a received signal is attenuated more for higher frequency signals than for lower frequency signals. As a result, the received high frequency data signal is smaller in amplitude than the low frequency voice signal. In a system which contains both voice and data on the same twisted pair wire, most of the dynamic range of the signal is used by the larger (i.e. less attenuated) voice signal. Typical interface systems incorporate a circuit to separate the data band and the voice band signals directly at the line connection point and use duplicate circuitry for each separate signal band. For example, U.S. Pat. No. 6,035,029 which issued Mar. 7, 2000 to Little et al describes a system in which a network interface unit at the customer premise separates the Plain Old Telephone Signal (POTS) from the data signal and each is processed separately. Circuits of this type typically contain an expensive transformer and the duplication of components in general adds to the system cost and power requirements.
Since the voice and data signals in such systems are full duplex (i.e. both transmit and receive at the same time on the same twisted pair wire), a fraction of the transmitted signals from both the voice and data bands appears across the connection points to the twisted pair. The connection point is also known as the Tip and Ring. These unwanted signals must be removed for proper signal reception. This is typically done in a voice only system using an analog circuit which subtracts the appropriate fraction of the outgoing (fraction transmit) signal from the incoming (receive+fraction transmit) signal. The resulting signal is the receive signal alone. This is known as a transhybrid loss circuit.
In a voice only system, since the transmit fraction of the incoming signal is in the same amplitude range as the receive component of the incoming signal, the transhybrid loss circuit function can be performed digitally after the analog to digital conversion without much compromise on the dynamic range of the analog to digital converter. This can be a large problem in a system which has a high frequency data signal, since the transmit fraction of the incoming signal is much larger than the receive component of the incoming signal.